Pumps for incompressible fluids, such as oil, are often gear, vane or piston pumps. In environments such as engine lubricating systems, gear pumps are often employed as they are reliable and relatively inexpensive to manufacture.
Gear pumps suffer from a disadvantage in that they are a constant displacement volume (capacity) pump (i.e.—they pump substantially the same volume of fluid for each revolution of the pump and thus deliver more fluid at higher operating speeds than at lower speeds). In environments such as automotive engine lubrication systems, wherein the pump speed will change while the required amount of fluid to be provided by the pump will remain substantially constant, the pump capacity is sized to provide the necessary volume of fluid at the expected lower operating speeds and thus, at higher operating speeds, the gear pump will oversupply the fluid.
To control the oversupply, and the resulting over pressure which would otherwise damage engine components, gear pumps in such environments are typically provided with a pressure relief valve which allows the undesired portion of the oversupplied fluid to return to a sump, tank or back to the inlet of the pump so that only the desired volume of fluid is supplied to the engine.
While equipping gear pumps with such pressure relief valves does manage the problems of oversupply at higher operating speeds, there are disadvantages with such systems. For example, the pump still consumes input energy to pump the oversupply of fluid, even though the pressure relief valve prevents delivery of the undesired portion of the oversupplied fluid, and thus the pump consumes more engine power than is necessary.
An alternative to gear pumps, in such environments, is the variable capacity vane pump. Such pumps include a moveable ring known as a slide ring, which allows the eccentricity of the pump to be altered to vary the capacity of the pump. Typically a control piston, connected to the slide ring, or alternatively, a pressurized chamber formed between the slide ring and the pump housing, is supplied with pressurized oil, directly or indirectly, from the output of the pump and, when the force created by the pressure of the supplied oil acting either on the control piston or directly on the slide ring is sufficient to overcome the force of a return spring, the slide ring is moved to reduce the capacity of the pump and thus lower the volume of the pumped oil to a desired level. If the supplied pressurized oil is at a pressure less than the desired level, then the force generated at the control piston or on the slide ring is less than that generated by the return spring and the return spring will move the slide ring to increase the capacity of the pump. In this manner, the output volume of the pump can be adjusted to maintain a selected value of pressure.
A disadvantage of both fixed and variable capacity pumps when controlled in the ways previously described is that, when operating above a threshold value of speed, the control pressure is constant according to the balance of forces between the spring and the pressurized area of the piston or slide ring. The threshold speed is the speed below which the pressure is insufficient to move the slide ring or open the relief valve. The value chosen for the control pressure depends on the worst case operating condition, which is typically at maximum speed, whereas the engine is likely to spend most operational time at lower speeds, when a lower control pressure would be satisfactory.
It is desirable in these circumstances to vary the output pressure of these pumps relative to the speed of the engine. Effective pressure control of the pump, based at least partially on the operating speed of the engine, can result in an improvement in engine efficiency and/or fuel consumption.
While such speed-related control can be achieved by a combination of electronic speed sensors, computer controllers and solenoid actuators, to date no effective and reliable mechanical means to accomplish such speed-related control has been available.